1. Field of the Invention
The present invention generally relates to a radio communication apparatus, a radio communication system, and a radio communication method using waveform characteristic data indicating signal statistic properties.
2. Description of the Related Art
In a radio communication system, when a radio station communicates, it is important for the radio station to control interference with other nearby radio stations. More specifically, it is required to prevent mutual communication interference between the radio stations by reducing the interference power level affecting other nearly radio stations to be equal to or less than the allowable upper limit value, so as to ensure a predetermined number of available radio stations in a radio communication system and prevent the degradation of the frequency use efficiency.
One exemplary method of controlling such interference is a frequency reuse method, in which the use frequency bands are designed to be allocated in advance in a manner such that the base stations adjacent to each other surely use different use frequency bands from each other. In this method, the base station is not allowed to use any use frequency bands other than the designated user frequency band determined in the design of the use frequency band.
FIG. 1 schematically illustrates the frequency reuse method. In this method, the use frequency bands are determined by centralized control for the entire service area of the radio communication system or a group having a predetermined number or more of cells. By using this method, it may become possible to sufficiently reduce the probability of causing communication failure. In the example of FIG. 1, the frequency bands f1 and f2 are allocated to the base stations 1a and 1b, respectively. Further, the base station 1a communicates with the terminals 2a and 2c, and the base station 1b communicates with the terminals 2b and 2d. In this case, a signal transmitted from the base station 1a arrives not only in the area of the base station 1a but also at the terminal 2b located in the area of the base station 1b. However, due to the different use frequency bands (i.e., f1 and f2), the signal from the base station 1a may not seriously interfere the communication of the terminal 2b. However, if this method is used, when the number of repetition is increased (i.e., the number of areas using the same use frequency band (hereinafter may be simplified as frequency) is increased) it may become difficult to perform flexible use of the frequencies, and the frequency use efficiency per unit area may be degraded.
There is another interference control method in which plural base stations are so worked together as to be virtually regarded as antennas disposed at geographically dispersed locations. By using the plural virtual antennas (base stations), the MIMO (Multi-Input Multi-output) communications (Multi-user MIMO) are performed; thereby enabling reducing the interferences and intending high-speed and large-capacity communication system (see, for example, D. Gesbert, M. Kountouris, R. W. Heath, Jr., C. B. Chae, and T. Salzer, “Shifting the MIMO Paradigm: From Single User to Multiuser Communications”, IEEE Signal Processing Magazine, vol. 24, no. 5, pp. 36-46, October 2007).
FIG. 2 shows a radio communication system in which the interference control is performed based on the multi-user MIMO concept. In this method, the interference control is performed while the plural base stations are worked together. Because using this method, it may become possible to sufficiently reduce the interference power level. Further, it is not necessary to reuse frequencies; therefore, higher frequency use efficiency may be achieved. In the example of FIG. 2, the base stations 1c and 1d are connected to a common network including a signal transmission/receiving control section. The signal transmission/receiving control section performs control on and determines the transmission signals of the base stations. By doing this, these base stations are virtually regarded as plural antennas which are disposed geographically dispersed locations. As a result, the same use frequency band may be shared by all the base stations and terminals. However, when the number of base stations is large, a large number of calculations are to be required to perform the control and the like. Further, in this method, very accurate information on the propagation paths between the terminals and base stations may be necessary. Because of this feature, a large scale of equipment change may be required to implement this method.
On the other hand, another method may be thought in which a terminal having received a signal collects information of the received signal, and anlyses a communication status. Then, based on the analysis result, the terminal determines a signal-transmission parameter to be advantageous for the terminal, and performs communications based on the parameter. Otherwise, there may be still another method in which the terminal determines a signal-transmission parameter to achieve a desired communication quality (i.e., desired qualities such as communication rate, error rate or the like), and perform communications based on the parameter. Herein, those methods may also be called environment-recognition type interference control methods.
FIG. 3 shows a radio communication system in which the environment-recognition type interference control is performed. In the example of FIG. 3, before the communications of the base station 1e and the terminal 2k, the base station 1e and terminals 2i and 2k detect and analyze the respective radio environments. More specifically, the base station 1e and terminals 2i and 2k monitor the respective receiving signals and detect available use frequency band. In this case, based on the respective received signals, if the terminals 2i and 2k can identify the transmission source in the signal component of the received signal, it may become possible to perform communication while accurately avoiding the mutual interference.
Namely, in the example of FIG. 3, the base station 1e and the terminal 2k receive a signal from the terminal 2j, but does not receive a signal from the terminal 2l because the terminal 2l is located at a geographically separated location. In the same manner, the terminal 2i does not receive signals from the terminals 2j and 2l. In this case, even if the frequency f1 used in the communications between the base station 1f and the terminal 2l is used in the communications between the base station 1e and the terminal 2k, it may become possible to sufficiently reduce the possibility of interference with the communications of the terminal 2l. Therefore, by using the frequency f1 in the communications between the base station 1e and the terminal 2k, the same frequency may be shared between base stations adjacent to each other; thereby improving the frequency use efficiency. Further, in this method, the radio communication parameters such as use frequency band is locally controlled in the base stations and terminals. Because of this feature, it may not necessarily required to perform a large number of calculations, and a relatively smaller scale of equipment change may be enough to implement this method.
In an environment-recognition type interference control method, it may become important to recognize where the radio station of the transmission source indicated in the signal component of the received signal is geographically located. To that end, in one method, the received signal at the terminal may be decoded to collect information of the transmission source. In another method, the information of the transmission source may be collected based on the characteristic data indicating statistic properties of the waveform of the received signal.
In the method of collecting the information based on the decoding of the signal, much information on the transmission source and the signal of the transmission source may be obtained at a time. However, when the power level of the signal to be decoded to collect the information is as low as to be buried in noise level, it may become difficult to collect the information. Further, under the conditions that the plural radio communication system are being used at the same time, the same number of decoders as that of types of the signal formats in the receiving signals may become necessary. Because of this feature, when the number of types of the signal formats is increased, the circuit scale of the stations may be accordingly increased. Further, when plural signals are received in the same frequency band at the same time, those signals may interfere with each other. In this case, even the signal power level is sufficiently greater than the noise level, the decoding process may not be properly performed.
On the other hand, in the method of collecting the information based on the characteristic data, the information of the characteristic data may be derived by calculating a predetermined statistics data of the received signal. In this method, even in an environment where plural signal formats are present at the same time, the information may be collected by using the same operation circuit. Therefore, this method may be advantageous because the circuit scale of the stations may avoid becoming too large. Further, this method may be advantageous because a signal buried in noise may also be detected by continually monitoring the signal at a predetermined time period, and because even plural signals are present at the same time, the information on the plural signals may be collected at the same time. The method of colleting the information based on the characteristic data in the signal is described in, for example, Japanese Patent Application Publication No. 2006-222665, where as the predetermined statistics data (value), a periodic autocorrelation (value) is used. However, in the method of colleting the information based on the characteristic data, an amount of the characteristic data obtained at one time is limited, and it may not be easy to obtain more information than that indicating whether the signal is detected or not upon the receipt of a signal. This is because, what can be obtained in this method is limited to the information extracted without decoding the received signal. Because of this feature, it may not be possible to extract information indicating “a parameter used in the signal (such as identification information of identifying the transmission source of the signal)”. Further, when plural transmission sources use the same parameter, it may be difficult to obtain information of those transmission sources.
In contrast, there is still another method capable of enhancing communication capacity of the entire areas, in which a radio station intentionally adds statistic properties to the transmission signal so that the waveform of the transmission signal from the radio station has a specific characteristic data; and plural radio station use plural characteristic data so that the communication capacity of the entire areas is enhanced (see, for example, Japanese Patent Application Publication No. 2008-061214, and P. D. Sutton, K. E. Nolan, and L. E. Doyle, “Cyclostationary Signature in Practical Congnitive Radio Applications,” IEEE Journal on Selected Areas in Communications(JSAC), Vol. 26, no. 1. pp 13-24, 2008). For example, by adding characteristic data dedicated to each station, it may become possible to determine the transmission source of the signal received by a radio station. As a result, by using this environment-recognition type interference control method, it may become possible to perform highly-accurate interference control.
However, in this environment-recognition type interference control method, as the number of types of characteristic data is increased, a recognition rate (i.e., probability to succeed in detection) of the characteristic data intentionally added may be reduced. Because of this feature, it may be necessary to limit the number of types of the characteristic data to be equal to or less than a predetermined number. However, when such a limitation is imposed, it may become difficult to allocate each dedicated characteristic data to all the radio stations; namely, the same characteristic data may have to be allocated to plural radio stations. If this is the case, as described above, it may become difficult to identify the transmission source based on the characteristic data. From the viewpoints of increasing the recognition rate of the characteristic data between the radio stations adjacent to each other, the characteristic data may be randomly added to the base stations. In this case, however, the radio stations to which the same characteristic data is added are disposed at geographically dispersed locations in various areas. Because of this feature, when signals are received at a certain point, the received signals including various types of (or all types of) characteristic data having non-negligible power level are more likely to be detected. In this case, if a radio station (observing station) tries to start communication at the certain point, the radio station may determine to refrain from starting the communications based on the detection that all the characteristic data may be being used.
FIG. 4 shows a case where various characteristic data C1, C2, and C3 are being used among various radio stations which are dispersedly distributed. There are many terminals distributed within the service area of the base station 1g. Further, the base station 1g communicates with the terminals 2m, 2n, and 2o using the use frequency bands f1, f2, and f3, respectively; and the terminals 2m, 2n, and 2o use the characteristic data C1, C2, and C3, respectively. Further, it is assumed that only three types of characteristic data C1, C2, and C3 can be used. Any of the characteristic data C1, C2, and C3 is allocated to each of the terminals within the service area of the base station 1g. Under the conditions, an observing station 3 observes the signals transmitted from the nearby terminals 2n, 2p, 2q, and 2r. Those nearby terminals 2n, 2p, 2q, and 2r use characteristic data C2, C3, C1, and C1, respectively. Therefore, the observing station 3 detects all the values of the characteristic data C2, C3, C1, and C1, to be equal to or greater than a threshold value.
In this case, the observing station 3 determines that any of the characteristic data C1, C2, and C3 cannot be used; namely, the observing station 3 determines that any of the use frequency bands f1, f2, and f3 cannot be used. This is because of the concern that if the observing station 3 determines to use, for example, the use frequency band f1 which is used in (allocated to) the communications between the base station 1g and the terminals having the characteristic data C1 within the service area of the base station 1g, the communications of the observing station 3 using the frequency band f1 may interfere the communications of the nearby terminals using the characteristic data C1 (i.e., terminals 2r and 2q).
However, the terminals 2m and 2o communicating with the base station 1g are disposed far from the observing station 3. Therefore, if it is assumed that the terminals 2r and 2q use not the characteristic data C1 but any of the characteristic data C2 or C3, the characteristic data C1 may not be strongly detected by the observing station 3. In this case, the observing station 3 may determine that, if the observing station 3 uses the frequency band f1 which is used for the terminals using the characteristic data C1 (i.e., in this case, the terminal 2m), the probability of interfering the communications of the terminal 2m may be relatively small. As described above, if the characteristic data and the use frequency bands can be appropriately determined (adjusted), the observing station 3 may start communication. However, in such a case as shown in FIG. 4, the observing station 3 may have to determine that any of the characteristic data C1, C2, and C3 (i.e., any of the frequency bands f1, f2, and f3) cannot be used. As a result, use opportunity of radio resources may be lost, which is not desirable from the viewpoints of effective use of frequencies.